The present invention relates to grates for use in industrial furnaces or the like, and more particularly to improvements in grates which comprise stationary and mobile grate bars, wherein mobile bars alternate with stationary bars and wherein each mobile bar overlaps in part a first stationary bar and is overlapped in part by a second stationary bar. Still more particularly, the invention relates to improvements in grates of the type wherein the marginal portions of outermost grate bars of each group of steps of the grate are adjacent to and cooperate with an expansible compensating device which reduces the likelihood of penetration of particles of solid matter between the frame and such marginal portions.
It is already known to provide a grate with compensating devices which serve to yield in response to heat-induced lateral expansion of adjacent steps of the grate and to expand in response to cooling of the adjacent steps. Reference may be had to commonly owned U.S. Pat. No. 4,096,809 granted June 27, 1978 to Martin et at. The patent to Martin et al. discloses and shows compensating devices along the lateral edges of a grate as well as between groups of steps of grate bars.
A drawback of many presently known compensating devices is that they cannot positively prevent penetration of particles of foreign matter into eventual crevices or gaps along the marginal portions of outermost grate bars. This is due to the fact that the purpose of compensating devices is to expand and contract in response to temperature changes. Penetration of particles of foreign matter between the mobile grate bars and the compensating device is not overly serious, at least under certain circumstances, because the mobile grate bars perform a self-cleaning action. Thus, if a particle of clinker, metal or fuel penetrates between a compensating device and the adjacent mobile (normally reciprocable) grate bars, such particle is likely to be squashed or expelled as a result of movements of grate bars with respect to the compensating device. However, any particles of foreign matter which penetrate between the compensating device and the stationary grate bars are likely to remain therein and to increase the width of clearance to thereby permit entry of additional (often larger) particles with the result that the width of the clearance increases gradually or stepwise. The width of such clearances can increase to such an extent that the compensating device is fully compressed and cannot yield in response to expansion of grate bars as a result of heating. Once such condition arises, further lateral expansion of grate bars results in development of stresses which are likely to affect the operation of the grate, to cause damage to component parts of the grate, or to cause partial or complete destruction of the grate. Thus, the compensating device becomes ineffective as soon as it is fully compressed prior to heating of adjacent grate bars to the maximum temperature. Moreover, the compensating device cannot expand to compensate for contraction of mobile grate bars as a result of cooling because particles of foreign matter which have penetrated between the stationary bars and the compensating device remain entrapped and hold the compensating device against expansion. The result is that the mobile grate bars are separated from the compensating device by extremely wide gaps which permit practically unobstructed descent of solid matter into the space therebelow. Moreover, when the pressure between the compensating device and the adjacent grate bars reaches a certain value, the mobile grate bars jam, i.e., they are incapable of reciprocating relative to the other grate bars. This interrupts or reduces the effectiveness of the aforementioned self-cleaning action of mobile bars. In fact, the pressure can reach a value at which the bars break or yield upwardly to develop gaps between the neighboring steps of the grate. Such gaps also permit solid particles (namely, the burning fuel) to pass through the grate. The particles cause further lifting of the raised bars and are likely to cause a complete breakdown of the grate. Excessive pressure between the grate bars and the compensating device further affects the operation of and/or causes damage to the mechanism which reciprocates the mobile grate bars.
As mentioned above, penetration of some solid particles between the compensating device and the mobile grate bars is undesirable but less damaging than the entry of such particles between the compenating device and stationary bars. This is due to the fact that the particles which are entrapped between the mobile bars and the compensating device are likely to be expelled owing to the aforediscussed self-cleaning action whereas a particle which penetrates between the compensating device and the stationary bars remains entrapped and facilitates the entry of additional foreign matter.